This invention pertains to a method and apparatus for providing particle-free air to dry a workpiece.
In manufacturing a cathode-ray tube for use in color television, a panel assembly is formed which includes an apertured shadow mask mounted in a faceplate panel adjacent to a cathodoluminescent screen. The screen comprises a pattern of red, green and blue phosphors surrounded by a black matrix on the inside of the panel. Phosphor slurries and a black matrix application are deposited on the inner surface of the panel utilizing chemical photoresist processes which use photoresist films. In forming the black matrix, wet coatings are applied to the panel's inner surface at various process stations, and need to be effectively dried. In particular, the panel receives a photoresist application, a preparatory solution to assist in graphite adhesion, and an application of colloidal graphite.
In drying each of these applications, it is important to prevent airborne particulate from impinging on the wet coatings, since particles in the air, particularly those greater than 0.5 micrometer, cause defects in the matrix pattern such as white or black spots. Airborne debris has also been linked to sag defects in the matrix coating. Sag defects are ripples in the resist or graphite coating that cause areas of lightness or darkness to occur in the matrix coating.
A uniform drying pattern is also important since a nonuniform drying pattern alters the photosensitivity of the resist film. Nonuniform drying causes a nonuniform amount of water to remain in the photoresist film. The amount of water determines the hydrogen ion mobility which changes the photochemical reaction. When the photoresist film is too dry, the images produced are smaller than desired, thereby causing a reduction in the allowable tolerance.
Faceplate panels are currently dried by portable horizontal air blowers using High Efficiency Particulate Air (HEPA) filters. However, airborne particulate continues to causes a low yield, resulting in a large number of panels being rejected. More importantly, what is a low level problem in entertainment tubes becomes a critical problem in high-resolution display tubes. Significant capital resources would be required to remove airborne particulate from the whole process room. In order to avoid this additional cost, it is desirable to be able to effectively clean critical drying stations along the production line rather than to provide a particle-free environment for the entire room.